Preheat oven to 175°C - 350°F - Gas Mark 4.In a medium bowl, combine the flour, baking soda, baking powder, salt and cinnamon.In a large bowl, beat the butter until fluffy, then add the sugars and vanilla. Beat until the mixture comes together and is no longer crumbly.Add the eggs one at a time, mixing well between each addition. Gently mix in the zucchini and lemon juice.Add the dry ingredients in two batches, mixing well between each addition, be careful not to over mix.Fold in the walnuts.Pour the batter into two dusted and buttered loaf pans and bake for about 40 to 45 minutes.Once ready, cool in the pan for 10 minutes, then turn out onto a wire rack to finish cooling.